The present invention relates to a method for manufacturing a microlens formed in a semiconductor substrate.
A microlens has been greatly used in optical devices such as an optical communication connector, a CCD (Charge Coupled Device) image pickup element, etc. Since the microlens is small in shape, a manufacturing method such as a reflow method, a gray-scale mask method or the like, which is different from grinding or molding used in the normal lens manufacture, has generally been used. The reflow method will now be explained in brief. First, a resist pattern approximately cylindrical in form is formed on a bedding or base substrate by photolithography. Thereafter, the substrate is heated to fluidize a resist, whereby a resist pattern approximately hemispherical in form is formed by surface tension. Subsequently, such a resist pattern is etched by anisotropic dry etching simultaneously with the base substrate, and the pattern approximately hemispherical in form is transferred onto the base substrate, whereby a microlens is fabricated.
The inventions related to microlens manufacturing methods have been described in, for example, patent documents 1 (Japanese Unexamined Patent Publication No. Hei 5(1993)-136460 (refer to the 4th page and FIG. 2) and 2 (Japanese Unexamined Patent Publication No. Hei 7(1995)-106237)(refer to the fourth page and FIGS. 4 through 7).
According to the microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 5(1993)-136460, a resist pattern approximately hemispherical in form, which is formed on a base substrate (GaAs) by potting, is used as an etching mask, and the resist pattern and the base substrate are simultaneously etched to fabricate a microlens (refer to a second embodiment).
According to the microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 7(1995)-106237, a plurality of first resist patterns formed on a base substrate in zigzags are deformed into hemispherical form by heat treatment. Thereafter, a plurality of hemispherical second resist patterns are further formed in spaces defined among the thermosetted or heat-cured first resist patterns. Thus, a CCD converging microlens whose lens-to-lens distance has been shortened is fabricated.
In the reflow method, the thickness of the post-reflow resist pattern, i.e., the thickness of its top portion approximately hemispherical in form, and the applied thickness of resist prior to the formation of the pattern by photolithography are placed in a predetermined proportionality relation. The thickness of the post-reflow resist pattern has heretofore been controlled by adjusting the applied thickness of resist.
With miniaturization of a lens and an improvement in its precision, however, a mere adjustment to the applied thickness of resist is becoming difficult to form a resist pattern necessary for desired lens formation. This is because demands contradictory to each other with respect to the resist thickness exist in two processes necessary for manufacture of a microlens, i.e., a photolithography process and a dry etching process respectively. In terms of processing of the resist by photolithography, there is a need to make thin the applied thickness of resist for the purpose of lens miniaturization. This is because optical transmittance is reduced upon exposure when the applied thickness of resist is thick, and a development remainder, so-called scum is apt to occur upon development. On the other hand, there is a need to ensure a predetermined thickness as the thickness of the resist pattern approximately hemispherical in form, which serves as the etching mask, in terms of the formation of the lens pattern by dry etching. It cannot be said that the thinning of the resist is a desirable direction. Thus, there is a need to increase the thickness of the resist pattern approximately hemispherical in form, which serves as the etching mask in the dry etching process, while meeting the demand for thinning of the applied thickness of resist in the photolithography process.
The microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 5(1993)-136460 needs not to perform the photolithography-based resist processing necessary for the normal reflow method because the resist pattern approximately hemispherical in form is formed by potting. Therefore, a description about the adjustment to the thickness of the post-reflow resist is not especially shown in Japanese Unexamined Patent Publication No. Hei 5(1993)-136460.
The microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 7(1995)-106237 has the fear that since the resist pattern approximately hemispherical in form is formed by the reflow method, a problem about such a resist thickness as already mentioned above arises with the miniaturization of the lens. However, the adjustment to the thickness of the post-reflow resist is not described in Japanese Unexamined Patent Publication No. Hei 7(1995)-106237 in particular.